The horn fly, Haematobia irritans (L.), has not been propagated previously in the laboratory beyond the first generation. It has not been difficult to obtain eggs from gravid wild flies, nor to rear rhem through to thc adult stage. However, a chemically defined diet that will promote ovary developrrient in reared adults has not been developed, although previous attempts at this laboratory (McLintock and Depner, 1957) showed that preserved blood is satisfactory as a diet to keep flies alive. Furthermore, the lack of sperm in the spermathecae of the females indicated that flies confined in various types of containers away from the host did not mate. Partial development of the ovarics of flies fed on a cow indicated that the method most likely to succeed was one in which flies were allowed to remain on the host.